The present invention relates to electrochromic devices which, in general, respond to varied levels of applied electrical voltage by changing light reflectivity or opacity. Such devices are particularly useful for providing visually-variable transparency or color in windows, mirrors, display panels, and the like. The invention is directed in particular to inorganic electrochromic electrolyte compositions, and incorporating devices, comprising aqueous solutions having electrically-responsive chromotropic species which may be repeatedly cycled between varied states of visual opacity or color display.
Currently-available electrochromic compositions and devices are for the most part prepared from either ion insertion materials or reversible electrodeposition materials. The former group includes such compositions as WO.sub.3 and Prussian Blue (ferric hexacyanoferrate) which respond to electrically-induced ion intercalation by changing color. Reversibility of such color change relies upon deintercalation of acquired ions in response to the reversal of applied electrical polarity. Consequently, the response time and state persistence for such compositions depends upon the ease of effecting the respective intercalation phases. Larger intercalated ions can thus severely restrict the practical utility, as well as limit the useful life span, of ion insertion devices.
Materials of the electrodeposition group utilize the electrolytic plating of metals, such as bismuth or copper, from solutions of their salts to achieve desired color or opacity change. As with the physical limitations affecting insertion materials, the ease of plating and dissolution of plated layers determines the response rate and reversibility of an electrochromic device based upon electrodeposition. Although increased gradients in applied electrical potential may achieve desired responses, components of the electrochromic device are often adversely affected with resulting loss of useful life.
Additional electrochromic compositions have been proposed which typically comprise exotic, costly electrochromotropic organic compounds and polymers. Such materials, however, in addition to their economic limitations, often exhibit distinct loss of chromophoric activity over time, thus making their commercial use unappealing.
Overcoming the "inertial" limitations of prior electrochromic compositions, that is, the natural resistance to large ion intercalation or to metal plate dissolution, is a prerequisite to the rapid response and long cycling life necessary for commercially acceptable devices. Economic considerations also contribute significantly to effective material applications. By avoiding such limitations through the use of variable phase inorganic solutions, the present invention provides a means for enabling low-threshold chromotropic activity as well as extended useful life in commercially viable electrochromic light valve or variable display devices.